This invention relates to a chemically aided waterflood oil recovery process. More particularly, it provides a process for compounding injected aqueous fluids having ionic compositions that minimize or control the amount by which those compositions are changed due to ion-exchange reactions with the reservoir rocks.
It is well known that chemical slugs, such as aqueous surfactant systems, perform most efficiently as oil recovery agents when formulated to contain specific kinds and "optimum" amounts of dissolved electrolytes or salts. Different surfactants or other chemical slugs require different amounts of salt to achieve their optimum performance. Further, it is known that the water in many, and perhaps most, reservoirs contain excessive amounts of dissolved salt. Most surfactant systems currently in use would not yield an optimum recovery efficiency if they were formulated in the water present in the reservoir being treated. Accordingly, it is common to formulate surfactant systems in low salinity waters obtained from lakes, shallow wells or other sources. These waters frequently require added salt to achieve optimum salinity and this added salt may be purchased as a commercial product and added with the surfactants. Alternatively, blends of fresh water and the saline formation water may be used to achieve optimum salinity.
Most reservoir waters and available fresh waters contain a mixture of salts. Frequently but not exclusively, the predominant cation in these waters is sodium. Some or all of the multivalent cations such as barium, calcium, magnesium, strontium, iron, aluminum, etc., are also present in these formulating waters. Iron, aluminum and other trivalent cations are normally present in insignficant quantities and, for the following discussion, will be assumed absent. It is well known that chemical slugs, such as anionic surfactant systems formulated to an optimum salinity can be drastically altered or de-optimized by the addition of more salt. Divalent cations are much more effective in de-optimizing typical formulations than are monovalent cations. An optimum surfactant system can typically withstand addition of as much as 0.25 to 1.0 percent NaCl without serious de-optimization. On the other hand, addition of less than 0.1 percent CaCl.sub.2 typically decreases the systems interfacial activity seriously. Thus, as little as 50 ppm increase in calcium or magnesium ion may detract substantially from a system's oil recovery performance.
It is also known that in chemical slugs, such as polymer-thickened aqueous solutions, the viscosity of the solutions, particularly those of polyacrylamides, are more adversely affected by small increases in multivalent cation concentration than by similar increases in monovalent cation concentration.
Various procedures have previously been suggested for optimizing the performance of aqueous fluids utilized in chemically aided waterflood processes. For example, U.S. Pat. No. 3,343,597 suggests preceding the injection of an oil miscible micellar dispersion by injecting an "insulating" water slug having an ionic content matching that of the water in the micellar dispersion. U.S. Pat. No. 3,467,190 suggests injecting an aqueous surfactant slug having an optimized salt concentration for providing a minimum interfacial tension against the reservoir oil, and preceding or following that slug with an aqueous solution of the same salt concentration whenever the salt concentration of the reservoir water or drive water differs from the optimum concentration. U.S. Pat. No. 3,482,631 suggests that where an oil-displacing fluid contains an electrolyte, the leaching or sorbing of the electrolyte, when equilibrium is established at the junction between the injected fluid and reservoir water, is reduced by injecting a preflood solution having a selected electrolyte concentration and an effective viscosity at least equalling that of the reservoir fluid. U.S. Pat. No. 3,648,770 suggests determining which cation is predominate in the reservoir water, and injecting a micellar dispersion containing a petroleum sulfonate surfactant and a cation which has a greater affinity for the sulfonate than the predominate cation in the reservoir water. U.S. Pat. No. 3,915,230 suggests preceding an aqueous surfactant system of optimum salinity and hardness with a thickened aqueous preflush solution of the same salinity and hardness.
In view of the adverse effects resulting from increasing the divalent ion concentration of flooding systems which are injected at optimal salinity, it is desirable to ensure that the systems traverse the intended portion of the reservoir without being subjected to changes in composition that exceed the selected design limitations. There are, however, several physical and chemical mechanisms which operate in typical reservoirs and invariably subject the injected systems to compositional changes. Among the more important of these mechanisms are crossflow of fluids between layers of different permeability, dispersive mixing within layers between the fluids being displaced and the displacing fluids, minerals dissolution when a water differing from the formation water contracts the reservoir rocks, and cation-exchange reactions between reservoir clay minerals and the injected water.
In general, it is believed that problems due to mineral dissolution are relatively minor. In cases where dissolution may be significant, the composition of the soluble minerals are known and ions designed to suppress the dissolution can be incorporated into various stages of the chemical flood process. Alternatively, the chemical slug can be designed to tolerate the compositional changes to be expected. Physical mixing (crossflow and dispersion) of the chemical slug with formaion water is generally believed to be a more difficult problem.
As previously indicated, for example, in patents such as those listed above, when the formation water is highly saline or contains relatively large amounts of multivalent cations it may be impossible, or at least uneconomical, to formulate an optimum surfactant slug having the ionic composition of the formation water. In such situations it has been common practice to formulate the chemical slug with a water of lower salinity and use a preflood or pre-slug to displace the formation water before the chemical slug is injected. Such prefloods have had various ionic compositions and/or additives to increase their viscosity, or modify the reservoir pH, or "condition" swelling clays, etc. But, in general, such prefloods have always been less saline than the formation water.